$B$-decay discrepancies after Moriond 2019
Résumé
Following the updated measurement of the lepton flavour universality (LFU) ratio $R_K$ in $B\rightarrow K\ell \ell $ decays by LHCb, as well as a number of further measurements, e.g. $R_{K^*}$ by Belle and $B_s \rightarrow \mu \mu $ by ATLAS, we analyse the global status of new physics in $b\rightarrow s$ transitions in the weak effective theory at the b-quark scale, in the Standard Model effective theory above the electroweak scale, and in simplified models of new physics. We find that the data continues to strongly prefer a solution with new physics in semi-leptonic Wilson coefficients. A purely muonic contribution to the combination $C_9 = -C_{10}$, well suited to UV-complete interpretations, is now favoured with respect to a muonic contribution to $C_9$ only. An even better fit is obtained by allowing an additional LFU shift in $C_9$. Such a shift can be renormalization-group induced from four-fermion operators above the electroweak scale, in particular from semi-tauonic operators, able to account for the potential discrepancies in $b \rightarrow c$ transitions. This scenario is naturally realized in the simplified $U_1$ leptoquark model. We also analyse simplified models where a LFU effect in $b\rightarrow s\ell \ell $ is induced radiatively from four-quark operators and show that such a setup is on the brink of exclusion by LHC di-jet resonance searches.